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MEXICO CITY (Reuters) - Immigration authorities in southern Mexico stopped a truck carrying 121 Central American migrants with signs of asphyxiation and dehydration, the National Migration Institute said in a statement on Thursday.

Migration officials stopped the truck during a routine operation in the southern state of Tabasco, and were alerted to the migrants inside when they heard screams for help. Of the 121 Central Americans they discovered, 55 were minors, with 23 girls and 32 boys.

The migrants had paid between $3,000 and $5,000 to be driven through Mexico to the U.S. border, and had been told not to eat or drink during the journey in order to avoid being detected by authorities, the INM said.

People smugglers have increasingly started using trucks to move migrants undetected through Mexico, which ramped up immigration detention efforts following a 2014 child migrant crisis on the southern U.S. border.

During fiscal year 2016, the number of unaccompanied children and family units detained at the southern U.S. border reached 137,366 people, exceeding the number in 2014, official U.S. data published on Tuesday showed.

"Border security alone cannot overcome the powerful push factors of poverty and violence that exist in Central America," Jeh Johnson, the Secretary of the Department of Homeland Security, said in a statement announcing the 2016 data.

"Walls alone cannot prevent illegal migration," he added. "The reality is the system is broken, and badly in need of comprehensive immigration reform that only Congress can provide."